Stable sulfur isotopes indicate net sulfate reduction in near-surface sediments of the deep Arabian Sea

Depth profiles of the stable sulfur isotopic composition of dissolved sulfate in near-surface sediments were measured at five stations of the deep Arabian Sea between 1918 and 4426 m water depth (WAST, WAST-Top, CAST, SAST, NAST), sampled in April 1997. The results clearly indicate that net microbial sulfate reduction took place in the sediments at stations WAST and NAST below about 12 cm depth. Sulfate reduction at WAST was more pronounced compared to station NAST, most likely due to higher organic carbon content in turbiditic sediments. No net sulfate reduction took place within the upper 10 cm of the surface sediments at all stations, and no significant isotopic indication for sulfate reduction was found down to 30 cm bsf at station SAST. Results are in accordance with accumulation of reduced isotopically light sulfur species below about 6 cm bsf at station WAST. It is concluded that the sulfur isotopic composition of remaining sulfate is more sensitive to net sulfate reduction than the [SO4]/[Cl] ratio. The sulfur isotopic composition of a vertical profile for dissolved sulfate through the water column at station WAST was essentially constant (250–4047 m: δ34S=+20.49±0.08‰ vs. V-CDT n=8). A similar constancy (20–4565 m water depth δ34S=+20.57±0.06‰ vs. V-CDT n=15) was found for the station BIOTRANS in the northeastern Atlantic (47°11′ 19°33W), indicating that the oxygen minimum zone in the Arabian Sea has no influence on the sulfur isotopic composition of dissolved sulfate.